How accurate is LMP dating?

Within about a week for regular 28-day cycles; ultrasound is more precise.

What if my cycle isn't 28 days?

Add (cycle length − 28) to the estimated due date.

Only about 4% of babies arrive on the exact due date; 37–42 weeks is normal.

Yes — prenatal care from early pregnancy is important.

Estimate your due date using Naegele's rule based on the last menstrual period (LMP).

Reviewed & updated: January 1970

First day of last period (LMP)Average cycle length (days)

Week

of 40

Estimated due dateMon Jun 07 2027

Estimated conceptionMon Sep 14 2026

Weeks along0 weeks

Results are estimates for general information only and are not professional financial, medical, tax, or legal advice. Read the full disclaimer.

Due date = LMP + 280 days (40 weeks). Adjust for cycle length ≠ 28 days.

Example

LMP Jan 1 → due date ≈ Oct 8.

How accurate is LMP dating?: Within about a week for regular 28-day cycles; ultrasound is more precise.
What if my cycle isn't 28 days?: Add (cycle length − 28) to the estimated due date.
Is 40 weeks always right?: Only about 4% of babies arrive on the exact due date; 37–42 weeks is normal.
Should I see a doctor?: Yes — prenatal care from early pregnancy is important.

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